In Real Life, a lock is a device for making sure an unauthorized person can't get through a specific door. There's generally anywhere from one to four keys made to fit a specific lock, and these keys usually won't fit in any other lock (unless the key happens to be a master key). The lock can be opened and closed over and over again.

In many games, however, any key will fit in any door. Sometimes keys will only work in the dungeon you find them in, but still, within each dungeon every key will fit every door.

Naturally, this would make opening all the Locked Doors very easy, except for one strange fact: whenever you use a key to open a door, the key, the lock, and sometimes even the entire door will disappear, right before your eyes — as if the key was made from antimatter. This makes locks almost useless for their real world purpose, which may explain why you only end up finding Locked Doors in dark forgotten dungeons beneath the earth and not on the doors to people's houses.

In a few games, the structure of dungeons may make it so that the interchangeable aspect doesn't ever come up; whenever you get a key, there's only one locked door you can ever reach, and the next key is behind that door. Still, rest assured that if you cheated up some more keys, or else got past a door without using a key, you'd find the game treats them as interchangeable.

Examples:

The Legend of Zelda series is a prime user of this trope. In fact, the first two games have a magic key whose only power is that it doesn't disappear when you use it to unlock a door.

The first few games have keys that work in any dungeon (the equivalent of the key for your front door unlocking Fort Knox). This creates problems, where you can become trapped if you use some keys from one dungeon that you didn't fully complete, in another dungeon. The first game has purchasable keys as a workaround, but the second doesn't and thus will require the use of the Fairy spell to get through the keyholes. Eventually, they simply made keys only usable in the dungeon in which you acquire them.

There are some keys that are required to enter the Boss's chamber (usually simply called the 'boss key'), and in The Legend of Zelda: A Link to the Past, there's the Big Key, which is reusable multiple times (within the same dungeon) to open any door with a very big keyhole.

Gauntlet has identical keys scattered throughout every level. Players can open doors or locked chests just by touching them if they have at least one key. The doors are particularly strange in that they can be extremely long and completely disappear the instant they are touched anywhere by a player who has a key.

In the Fire Emblem series, there are 2 kinds of keys: door keys and chest keys. Any door key can open any door, and any chest key can open any chest. If a particular game has lockpicks, they open both. Shadow Dragon has two others: bridge keys (guess) and master keys (open anything, and have five uses each).

Wild AR Ms called these keys "duplicators" and explains that the key could duplicate into the specific key to fit any lock, once. Afterward the transformed key is no good for any other door, so is thrown away.

All keys in the game are identical and can be used to open any locked chest in the game. Yes, they disappear afterwards. The only exceptions are the special keys that are sometimes dropped by the Countess, the Summoner and Nihlathak on Hell difficulty. They don't open anything, at least not in the conventional way.

A common feature of mods for Diablo II were some manner of gambling-related chests containing magical items. Usually the keys were not just interchangeable but didn't even open the box at all — they had to be transmuted together in the Horadric Cube (although this was generally for technical reasons; Diablo II didn't have any code that allowed for a container item in the traditional sense).

While the actual keys behave completely like normal keys, its reconfigurable nanolockpicks and electromagnetic multitools fit this trope like a glove, although they do have somewhat reasonable excuses as to why they're non-reusable. Although there's not much of an explaination for why it can take four to six lockpicks to unlock a door, while the lock weakens with each try.

Then there's keyrings, which in Deus Ex are strange pistol-shaped tools that rapidly reshape small bits of nanites into the form of keys. To do so a keyring need a nano key, a small blue cylinder containing said key's dimensions. Unlike this trope however, with both in hands, the door can open or be locked again at your leisure.

The third game has the single-use Automated Unlocking Tools, which unlock any system, be it a security station or a keypad lock. There are also Stop! and Nuke! viruses on floppy disks which can be used on any system and cannot be reused.

In Landstalker: The Treasures of King Nole does a mix. You carry mostly Interchangeable Antimatter Keys, except when unlocking dungeons. For example: you unlock the heavy stone door of dungeon #2 with an...idol? No, you don't slot it into an empty pedestal, you just "Use" the Idol.

You only ever find the key you need in Drakengard, but some instances of this are ridiculous. For example, the first stage requires you to find a key to open the castle's portcullis, despite the fact that you don't lock those things with keys, if at all.

Justified in Betrayal at Krondor, in which every item you can pick up has a paragraph-long description (mostly fluff). The description of the standard key says that it's made for a cheap, low-end lock, "probably manufactured by the hundreds", and "likely to snap". Keys (and lockpicks) have a chance to break and disappear when using them, though they don't vanish every time. More important locks have unique keys, which being less cheaply made are fully reusable.

Played straight in the Eye of the Beholder series as well as its spiritual successor Lands of Lore (at least the first one). The first Eye of the Beholder game was notable because each level had multiple keys and each could be used interchangeably on most of the level's locks. Since some locked doors eventually connected to the same area it paid to only unlock doors when you needed to (thereby saving the trouble of looking for additional keys). Furthermore, examining a lock afterwards would reveal that it was jammed, justifying the key being destroyed (it got stuck inside the lock).

The keys in Lands of Lore aren't interchangeable, but they are anti-matter. Also worth noting is one particular dungeon, the White Tower; while most keys in the game are noticeably unique in appearance, this area uses special "Mystic Keys". These keys are all shaped exactly alike and are only differentiated by their color, but since nothing in the Tower is color-coded, figuring out which key goes into which lock is a matter of trial and error.

Video Game/{{Dragon Quest I has one-use keys, though later games in the series allow single keys to open multiple doors (and they are magic, which explains one key fitting every door in the world). Then you run into Fridge Logic in Dragon Quest III, the prequel to the first game. You take your magic key, which can be used infinitely but doesn't open every door in the game (you need the Final Key for that), and show it to the guy who, by Dragon Quest I, is selling magic keys. Presumably, he copies it, but makes them one-time use and capable of opening any door in the world. In other words, his copy attempt failed completely.

Keys in Boktai work this way. Most dungeons have a square key for the square key door, a circle key for the circle key door, etc., but Otenko notes when you first pick one up that the keys were made by an Immortal and will evaporate if you take them to another dungeon.

Weights from La-Mulana qualify. They are found everywhere and can only be used once. They can be placed on pedestals throughout the ruins to activate things. Things like doors, ancient mechanisms, and when you least expect it, Death Traps. The Ankh Jewels also qualify, as you don't need to use the jewel from the same area to activate that area's boss.

The Pokémon Mystery Dungeon games are guilty of this as well. In the first games, even if you have a key, it may not be enough to get to the treasure. Sometimes, you'll need a certain move or terrain ability as well.

In Final Fantasy IX, the doors in Burmecia are opened with bells, of all things. Moreover, those bells shatter after being used for no adequately explored reason. Since some of these doors lead to fairly important areas, such as the palace and the place people get married, one must imagine that somewhere a government contractor is raking in the gil making disposable bell keys.

In Final Fantasy III, you can only open certain locked doors if you have a "Magic Key" (which disappears after use) or a Thief in your party, who can pick the lock. This rarely comes up outside of Goldor's Mansion, though.

The Atelier series often uses a common form of this trope with color-coded chests that require keys, with the rarest chest type needing a key you'll only have access to near the end of the game. Using the key will cause it to vanish, but once you have the key's recipe you can alchemize as many as needed.

ZZT has a variant of this: Keys and doors come in seven colors, and any key of a given color unlocks any door of that color (and then disappear forever). You also can only carry one key of each color at a time, which leads to situations where your way might be blocked by a key (which is the same size as the player, the doors, and everything else) that you cannot pick up until you find a door to use the key you already had on.

The Subspace Emissary mode from Super Smash Bros. Brawl has one-time keys that open doors this way. They respawn in their original locations if the key is lost or abandoned in the way, but they're gone forever after they're used.

The two 3D Klonoa games play this straight ("One key, one door") although it's the variation in which you can only reach one key at any time. The 2D puzzle games avert it, however: keys come in multiple shapes, and each one can open all of the matching doors in that level.

Bugdom, although the cheat codes are so ludicrously simple (Shift+Tilde+F1-F9) that it's really not that hard to just cheat your way into a complete set.

Neverwinter Nights. The module editor lets you make each door work however you want, with the keys being interchangeable or not as you desire. You can also set the "relockable", "requires key to unlock" (as opposed to being vulnerable to the Open Lock skill), and "consumes key when used" flags, although not all three at once.

The game not only has this trope in effect, but the keys (with one exception) have to be bought from a village locksmith. There are, however, five different types of keys.

Xanadu Next uses a similar system; Standard keys are actually magical skeleton keys carved from monster bones. Most of your keys are going to be bought from a store at your home base, but you need to sell him more bones that the shopkeep can carve keys out of, or he'll have to drive the price up with each one you buy. You can also carve your own keys when you find a carving knive.

In Little Big Adventure games, keys are intended for doors located in the same area, and when you leave that area without using the key you found there, it'll disappear. However, all keys are identical, as demostrated in areas where there is more than one door to open - getting one key will give you access to any of the doors you choose, but it vanishes after use, and you'll have to get more keys for other doors in the area.

Dubloon features keys that disappear from your inventory the moment you drag them into locks (which aren't doors so much as flat blocks with keyholes in them).

Team Fortress 2 has this in the form of Mann Co. Supply Crate Keys. You can get locked crates from the Random Drop System, which in turn can be opened by these one-use keys. The problem? You have to buy the keys. With real money, or if you are cheap or have little money you can get them also by trading metal, hats, event items (Halloween, etc.), or strange weapons. VG Catsexplains. There are also special keys for Seasonal items/Promotional events which only open special crates, those keys turn into normal ones after the event ends.

Lost Odyssey features this in one dungeon with "gate pass" items you have to steal from enemies to deactivate some electric gates. All the passes fit all the gates, and are consumed in the process.

Secret Agent has the classic set-up. Colored doors (red, blue and green), colored keys, doors disappear when you walk into them with a key. On some levels it's possible to pick up a key when you've already got one of the same colour, which causes you to waste a key and inevitably makes the level unwinnable.

Pocky & Rocky 2 has locked chests and occasionally locked doors that can all be opened with identical keys. Keys can be found in baskets, dropped by enemies, or bought in stores, but if you have Little Ninja as your partner, you can use her magic to pick the locks and bypass the need for keys.

Wonder Boy III: The Dragon's Trap is an egregious example, as the locks actually look different, but are all opened with the same keys. (The different locks indicate whether a door has to be unlocked just once or every time you open it.)

That Legacy of the Wizard displays a meter for keys right next to the life, magic and gold meters should show how much this game relies on feeding keys into doors. Fortunately for Meyna, whose area contains long strings of locked doors, there's an item that lets her use magic instead of keys to open doors.

In Spyro the Dragon, Spyro only ever needs one key at a time, located a sufficient distance away from the locked box to make things..."fun". Spyro: A Hero's Tail had things called "Lock-Picks," which you had to buy so that your little dragonfly friend could fly into locks and pick them, but they looked more like keys than like actual lock-picks.

In the first game, a key can open any locked door or golden chest. After using, it disappears.

Wrath of the Lamb added special Golden Keys, shaped like skulls, which can open an unlimited number of doors and chests...but for that floor only. You lose it once you go to the next floor down.

You can also pick up a trinket that randomly opens doors and chests without consuming keys.

Dad's Key is an unlockable, rechargeable item that, when used, will open any closed door in the room it's used in. No matter what kind of door this may be. This even includes the Giant Golden Door that leads to the True Final Boss, which otherwise needs a very specific key that must be found in two parts to open.

Tutankham had keys disappear when used, though the player could only carry one key at a time.

Flink has single-use magic keys for opening locked treasure chests. They are selected through a menu like other spells which work quite differently.

Terraria has a straight example in the golden keys for locked golden chests. They also have a slightly different version in each type of key in the game. Shadow keys aren't consumed. Temple keys open only one door in the game, but they can be crafted with rare molds into other keys. These keys are specific to one chest which may not exist in every world, and for some reason cause every copy in your world to be consumed.

Dust: An Elysian Tail has locked chests and cages (housing characters from other indie games) that operate on this principle: keys are scattered throughout the game world, and any one key will open any one lock (with cages having four locks each), only to vanish on use. They keys and locks are supposedly magical, though, which might explain the mechanic.

Hans Kloss: There are lots of this kind of keys scattered around. Half of the challenge is figuring out (via Trial-and-Error Gameplay) which doors to open at which time, to avoid getting stuck.

In Rock 'n' Roll, a Mad Marble Maze game by Rainbow Arts, keys come in four colors, with each key canceling out one door of the same color.

Festers Quest is even worse about it than most games. Doors don't remain unlocked after spending a key on them, so you need to use up another key if you come back later. This is because keys are a Random Drop.

In Pixel Dungeon, keys are only interchangeable if they correspond to locked rooms on the same floor. A key from floor 3 won't unlock a locked door on floor 4, for instance. One Golden Key can open any Golden Chest on the same level.

The first Fragile Hearts plays it straight in the castle. Every time you visit it, the keys you collect will unlock any currently locked door, and then disappear from the inventory. One door is even bugged and wouldn't stay unlocked, and going back there can actually render the game Unwinnable by Mistake. On the flip side, the Throne Room does require a unique key (which will also disappear after being used.)

In Dustforce, a single-use key will work on any appropriate door of that type.

In Wizorb, having the key somehow lets you unlock a door simply by touching it with a seemingly unchanged orb you control. The key will disappear after that procedure as well.

Done in Another Perspective, where any key will open any locked door, and the key will promptly disappear afterwards.

These appear in the "Nightmare" mode of Back To Bed. Subob needs to ensure that Bob’s path will overlap with all of the keys lying around on the level, so that the door to his bedroom will be unlocked at the end.

In Live A Live, the final chapters's Dungeon of Keys has locked doors that need to be opened with Rusted Keys, which are randomly dropped by a specific enemy and crumble to dust when used.

Ironsword: Wizards and Warriors II had chests locked by these keys, but it was the only game in the series to use this trope. Wizards & Warriors had three differently colored keys that could be re-used for the whole level, and Wizards & Warriors III had a skeleton key that could unlock anything.

In Golden Axe Warrior, the player has to rely on generic single-use keys to open doors until the Thief's Key, which can open an unlimited number of doors, turns up late in the game.

In Hollow Knight, any locking mechanism described as "simple" can be opened by any Simple Key item. However, all Simple Keys are described as being so poorly crafted that they break after just one use.

MDK features locked doors that are unlocked by... throwing mini-nukes on them (that somehow don't pulverise anything other than the lock). The "keys" unreusability definitely makes sense in that case.

In 8Bit Killer, interacting with a Locked Door with a key in your inventory unlocks the door and instantly removes the key. The interchangeability of keys comes into play in stage 2-2, where using the key you find on the wrong door(hint) It's not the first one you see when you enter the entrance hall. will lead you to the exit but also lock you out of an assault rifle.note Next to this assault rifle is another key, so no worries.

Since a key allows accessing a specific player house, it is useless for anything but one specific door. More keys can be made for the same door by copying the pattern from an existing key to a "blank key".

Ultima VII: The Black Gate, from which Ultima Online largely derived its style, also featured a unique key for every locked door or chest in the game.

Ultima VII Part II: Serpent Isle had even more keys. Thankfully its add-in pack Silver Seed introduced a keyring where all keys could go. The keyring could then be used directly on a lock and if would open if the key was on the ring. Ultima Underworld II: Labyrinth of Worlds had already introduced a keyring but in that game the keys still had to be tested individually.

Addams Family Values also had several dozen unique keys to collect, a few of which don't even have a door to use them on.

Luigi's Mansion has door specific keys for the whole mansion and never to be used again. In the sequel, keys explicitly disintegrate when used. Early dev versions featured door locks that ATE the keys.

Fable I had a variation on this trope with its Silver Keys. Any silver key can be used in any silver chest and are used up when the chest is opened, but chests require multiple keys. It doesn't matter which keys you use, so long as you have more than the key number of the chest.

Final Fantasy I featured several items used to grant access to various dungeons that did not disappear when used, even if they only needed to be used once. It also included many doors that were described to the player as "locked with the MYSTIC KEY" — once the player obtained said key, they could access the doors.

Sly Cooper, too, featured the collection of keys that did not disappear when used. Keys could only be used in the general world zones in which they were collected, and specific plot items and doors were locked with a number of locks that determine how many keys Sly needed to collect within this zone in order to unlock the next part of the plot.

Zig-zagged throughout the Resident Evil games. The first two games have small keys that are used to unlock desk drawers and the like and they vanish upon use. There are also keys that can be used to unlock several doors and once all the doors associated with that key is unlocked, they key has no purpose and you can discard it if you want to.

Silent Hill also subverts the "every door opens" aspect by including many, many doors that can never be opened at all. If the lock is jammed, it stays jammed forever. In fact, this is such a staple of the Silent Hill series that Silent Hill: Shattered Memories has the ability to open any door as a selling point.

The webcomic Kid Raddlampshaded this with Itty Bitty, a shopkeeper NPC, who had an unlimited amount of such keys, and used them several times to escape imprisonment.

Lufia and Lufia II avert this trope by having unique keys for every door, and after they're used the doors stay open forever and the keys stay in your inventory.

Lunar: The Silver Star has the ever-obnoxious red chests that can only be unlocked by a specific item much later in the game...and it's not always worth it.

In Arcanum: Of Steamworks & Magick Obscura, each key is can only be used on specific locks, and continue to exist once they have been used. Due to this, an accomplished thief (or brutal murderer) can collect upwards of 30-40 keys by the end of the game. (They take up a bit of room until you get a key ring, though...)

Played straight with the lockpicks in Morrowind and Oblivion; in Morrowind, they break after a certain number of uses, and in Oblivion, they break if you fail at the Lockpicking Minigame. There are also Open Lock spells with varying magnitudes, but some (usually plot-important) doors require keys to open.

Morrowind also has a quirk with this involving slave bracers. Normally, you need to find the key in the location with the slaves in order to unlock their bracers. (So, for example, the Addamasartus Slave Key will open the bracers of slaves within Addamasartus.) However, if you bring a slave from another location to a place for which you already have the key (using a Command spell, for example,) you can then unlock that slave's bracers with the key you already have. This is useful for freeing slaves whose bracers do not have a key and normally would not be able to be freed.

Skyrim also has the breaking lockpicks, and one could go through a multitude of lockpicks on one lock, depending on the lock difficulty. However, once you get your lockpicking high enough and get the required perk, or you aquire the Skeleton Key, no longer will you have to worry about breaking lockpicks, especially on Master level locks.

All Commander Keen games (except Keen Dreams, which has only one type of key) have several colours of keycards or keygems that open a door of the same colour. In case of the gems the gem doesn't actually disappear, it just becomes stuck to the pedestal you place it on.

The skeleton keys of Nethack don't disappear on use, which makes them all the more useful - they work on every door and box in the game. The variant Slash'EM occasionally has these keys break in the lock, but eventually you can find up to three unbreakable artifact keys, each one of which unlocks almost everything in your way. The few things they can't unlock...well, that's why there's three of them.

The NES game Castlequest had their keys and doors color-coded. Figuring out how best to spend your keys was part of the challenge.

Paper Mario games both play this straight and avert it. A few dungeons (such as the final dungeon in the original and the fifth dungeon in The Thousand-Year-Door) use Interchangeable Antimatter Keys, but in others, you get specific keys which only work in certain places, and some of which may stay in your inventory after use. The "a specific key will only work here" places are generally in the overworld, but many dungeons also have sequence puzzles solved by using specific keys in certain places.

Beyond Good & Evil has a grand total of four keys in the game. Two of them, the Triangle and Square Keys, are used multiple times across multiple dungeons and side-quests. The third is a duplicate Triangle Key which is needed for one dual switch puzzle; it's a bit pointless after that, but can still be used to open doors. The fourth, the Star Key, does become useless after one sidequest, but never vanishes.

In Chip's Challenge there are four key colours. Red, yellow and blue keys vanish when used, but green ones don't. This has the effect of making certain puzzles Unwinnable by Design if the player opens certain locks at the wrong time (for example, opening the doors to the chip door or the goal in Elementary before getting all the chips behind the other doors).

Kingdom of Loathing is mainly an aversion, with an extremely weird subversion- there are a few different keys, each used in a specific place- for example, Boris's, Sneaky Pete's and Jarlsberg's Keys open their Shrines in the Big Mountains. Played straight in the Naughty Sorceress Quest, as the keys now disappear when you use them to open the door.

Also lampshaded upon using a certain key, which gives you the message "In typical video game fashion, the key vanishes after having been used once."

This is played straight with numerous items that are not technically keys, such as dolphin whistles. They invariably get "knocked out of your hand" and cease to exist after a single use. Can't you just tie the whistle to your wrist or something?

A key or keycard will only work with specific doors, and you can keep the keys afterwards. You can even find keycards which have no use in game, or find keys well before (or after) you find the door they're used on. Oddly, you can sell some keycards afterwards.

In Fallout 3 and Fallout New Vegas, instead of keys you get lockpicking tools, comprising of bobby pins (that can break in a failed lockpicking attempt) and a screwdriver. They can open nearly any lock except for the very rare unpickable ones (that are usually there for Rail Roading purposes), meaning the same tools that can open an ammo box can also open a sturdy safe, or even the high tech security door of a bunker!

Fallout 4 has similar lock mechanics to the previous two games. The locks have specific levels, and require you to level up and apply perk points to the Locksmith perk to access the more difficult locks. Maxing out Locksmith will make your bobby pins unbreakable. Terminal-locked security doors work on a similar principle, with the Hacker perk being required to unlock higher-level terminals; maxing out the perk keeps you from being locked out (where you need to wait a certain interval before trying to hack it again). There are still certain locks and terminals that require specific keys and passwords, though, due to the requirements of certain quests, though once you've acquired them, you keep them.

World of Warcraft has just about every sort of version imaginable. Many dungeons have doors that require a specific key to open, and that key, once obtained, remains with the player forever unless discarded. Certain "keys" don't actually open a door but instead act as Plot Coupons that unlock harder difficulty modes. Still other dungeons have keys that only exist within that dungeon and must be obtained every time the player enters; these are consumed once used or if the player leaves. Many, many quests involve obtaining keys that can only be used for that quest and are consumed as they are used; leftover keys also spontaneously vanish once you complete the quest. There are randomly scattered chests and lockboxes throughout the game; these can be opened by a Rogue with lockpicks, Engineers with blasting charges, or Blacksmiths with skeleton keys. Those same methods may also be used on many—but not all—doors (with no particular explanation for the difference), and only the Rogue's is reusable as it doesn't involve a consumable item. Ironically, the cost of crafting a blasting charge or skeleton key is frequently greater than the value of the items you obtain from using them.

In Wizardry VII all keys self-destruct (some has several uses, some one) upon unlocking, but there's mix of "unique" and several "common" keys (latter can even appear in random loot).

Quest for Glory IV: Shadows of Darkness sees The Hero, Prince of Shapeir hold on to every key he finds because they only open specific doors and locks. There are only four keys you can get anyway (your inn room key, the key to Dr. Cranium's lab, the crypt key, and the key to the adventurer's guild).

In EarthBound, after a specific key is found to be too bent to use, the person who gave you it exchanges it for the "Machine that Opens Doors, especially when you have a slightly bad key". This opens the lock in question, as well as another plot-required lock shortly afterwards, and then has no further use ever again. Since it sounds incredibly useful, many first-time players likely keep it in their (very limited) inventory for the whole game.

Keys in Geneforge are rare and open specific doors, but "living tools" (lockpicks) vanish when used. However, they're a "deviant use" in the sense of being justified—they're Organic Technology, and they die after being used.

Most MUDs have locks and doors that can only be opened by a specific key (unless you can pick the lock).

Maniac Mansion handles its keys very realistically, although this is helped by the limited number of doors and locks in the game. All the locks are differentiated and require different keys, although there are locks keyed alike (such as the coin boxes in the arcade room), all of the opened locks can be re-locked and all of the keys stay in your inventory (which can be kind of a pain, given that there are three different inventories for three different kids).

Jagged Alliance games have plenty of keys to be found. The original Jagged Alliance 2 campaign has over 100 different sets of keys to be found, and each unlocks one or more doors, so they never disappear and it can get confusing. Some keys have duplicates, and some are useless. However, it is impossible to lock a door once it's been unlocked, and enemies are able to pass through locked (or trapped) doors without any key, effort, or lock-picking equipment. Another interesting aspect is that the character opening the door needs to be carrying the key (no shared inventory), which can be very frustrating during combat.

RuneScape is an aversion, in that nearly every door has a unique key associated, and most of them are infinitely reusable. Fortunately, there's also a keyring item which lets you store certain important keys so you don't have to waste nearly as much bank space as you might think. At the same time, it is present and accounted for in a lot of cases in the game as well. Crystal Keys break off in the lock, as do H.A.M. keys, but Shade Keys go one step further by dissolving when used.

Redline didn't even bother with keys at all. In one mission, after coming across a locked door, your Mission Control flat out tells you not to waste your time searching the level for a key and to just blast the door down with a rocket launcher.

Braid subverts this quite cleverly. The keys are indeed interchangeable and single use...but you have time traveling powers. Some puzzles require a specific key be used on a specific door even though they appear to be interchangeable (In World 3, time moves backwards as you walk - try to unlock the wrong door and time reverses it as you walk through, locking the door again...but your key stays disappeared), while others need you to open multiple doors with a single antimatter key.

Wolfenstein 3D plays around with this. You can open any locked door in a map as often as you want after you find the necessary key. However, once you exit the level, you lose all your keys and usually have to find them again for the next map. This is also the case in Doom and severalgameson the Build engine.

Justified in Ōkami, where the keys in question are more like holy arrows, and the doors have a monster attached to them. Said monster can only be destroyed with the key/arrow, and the resulting explosion destroys the door. (Of course, the monster looks like a lock.)

The Tower of Druaga sort of justifies this by requiring a potion instead of a key to open locked chests.

Tibia has each key assigned to a specific door and not only that, the keys were reusable as well. You could freely lock doors you've just unlocked. In some cases, a key won't even be necessary to access an area because some other player may have left the door unlocked. It'll be risky to enter, though, as some other player may decide to lock you in with your only hope of escape being relying on the kindness of others, death, or waiting until someone else leaves the door unlocked.

Most Point-and-Click Escape-The-Room adventures will have multiple keys that are definitely not interchangable (since this would obviously lead to massive Sequence Breaking, such as using the first key you find to open the main door), but will disappear from your inventory as soon as you use them.

In Vampire Killer, each small key was only good for opening any one treasure chest, though you could sometimes uncover or purchase a lock pick that could open three treasure chests. You could only carry one of either at a time. You needed a different kind of key to open exit gates, but there was never a choice between those.

In Divine Divinity, each key is unique, all locked doors require a specific one, making lock-picking one of the most essential skills.

French MMORPG Dofus has an odd variant. Most keys can only be used once to enter dungeons, but that's because the person standing at the entrance to the dungeon explicitly takes the key from you. (One castle is an exception, where you have to find three keys, but they're quest items and you never lose them) There's also key rings which can get you into any dungeon in the game and are never lost, but which for some reason take a full week to "recharge" before you can use them on that particular dungeon again.

Outcast has a rare case of providing an in-game explanation for one-use keys. Lockable doors in the setting are rare, so a small farming village only really has need for one locked storage. With a tyrant in charge, the soldiers have taken over the use of any such spaces for their own needs. Meanwhile, there is a popular resistance movement and one person who crafts keys mentions he makes them deliberately brittle so they break with one use and the soldiers will lock themselves out from their own supplies. The protagonist reacts with appropriate frustration upon hearing this.

Averted in Thief. Keys act like actual keys: many times, a key will open multiple locks, and it can be reused as many times as you like. In fact, you can re-lock the door by using that key on it again. Many guards will also carry copies of the basic keys, which makes sense given their job. However, a true Skeleton Key is rare: often, particularly valuable stuff is behind a unique lock that requires a unique key. And even if you don't have the key, many locks can be picked... although there are some that cannot, and require the key to continue.

Subverted in Mystic Towers. Keys will disappear after being used, but they aren't interchangeable, which makes them into a prominent gameplay element. Every room requires a key to unlock its door first, and often, they are not even on the same floor, requiring lots of back-and-forth traversing around the place. Finally, the tower door itself locks behind you and you'll need a large red key to leave.

The Metal Gear series has keycards. Later games make them work automatically whenever you walk near the door they unlock without even needing to be equipped. Not so with the games preceding Solid, and when you start having around eight separate keycards to juggle through while simultaneously denying you access to your standard inventory...note One memorable instance in the NES game requires you to remove your gas mask inside a gas-filled chamber just to equip a keycard and exit.

The Another Code games avert this, due to finding the right key for each lock being a common task for any non-puzzle-locked door. The keys are generally removed from your inventory, but mostly because Ashley has no reason to relock the doors. The most notable time a key is used on more than one door is a puzzle in the first game that requires how you use two keys to open it.

As they take place in a hotel and apartment complex respectively, it's no surprise that Hotel Dusk: Room 215 and Last Window requires gathering specific keys to open the right doors. The keys also remain in your inventory after being used, like every other inventory item that isn't automatically used up.

Blake Stone averts this and combines it with Bag of Spilling. On each level, various keycards will unlock doors. As you advance through the game, you find that the keycards from one level don't work on the others; as an Anti-Frustration Feature, the game does save which keycards you have on each level in case you decide to backtrack and chase 100% Completion.

Rolo to the Rescue averts this. Having one key allows Rolo to unlock every cage in its level.

Subverted in It's Always Sunny in Philadelphia when Dennis and Charlie are attempting to open a door at a competing bar and restaurant. Upon discovering it is locked, Charlie attempts to use his apartment key with the reasoning that it works on his apartment door, Dennis responds angrily "We're not at your apartment shithead!". As expected, the key does not work.

Over the course of Cave Story you find multiple keys, each of which opens one specific lock and stays in your inventory long after fulfilling its purpose (so they are neither interchangeable nor antimatter).

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